1. Field of the Invention
The present invention relates to a wireless communication network, and more particularly to a method and apparatus for efficiently controlling buffers which support a Hybrid Automatic Repeat Request (HARQ).
2. Description of the Related Art
Recently, mobile communication services have been tested for the possibility of creating a new market while providing various services, such as broadcasting, multimedia images, e-mail, multimedia messages, etc. In such an information-oriented age, the demand for various wireless multimedia services of different qualities, for example, from a low speed to a high speed, in either real time or in non-real time, etc., has been increasing.
For this reason, studies are being conducted to develop new technology for efficiently allocating users with frequency channels, i.e., limited frequency bands, which are a main resource in a mobile communication system. In line with this, radio transmission element technologies, such as wireless multiple access and multiplexing, high-speed packet radio transmission, and a radio link control, have been proposed in the wireless communication system.
Especially, in the radio link control technology, a Hybrid Automatic Repeat Request (HARQ) scheme is technology for controlling errors, in which an Automatic Repeat request (ARQ) scheme and a Forward Error Correction (FEC) scheme are combined. The HARQ scheme can be applied to packet data services for bursty packet data, such as wireless Internet packet, that is, to services requiring a high reliability of transmission data.
A receiving terminal employing the HARQ scheme transmits either an affirmative signal, i.e. an ACKnowledgement (ACK) or a negative signal, i.e. a Negative AcKnowledgement (NAK) to a transmitting terminal as a response signal according to whether or not received data has been successfully decoded, thereby requesting the transmitting terminal to retransmit the same data if necessary. That is, the HARQ scheme has such a mechanism that the transmitting terminal retransmits corresponding data when receiving a NAK signal as a result of decoding from the receiving terminal. The receiving terminal obtains a gain in the reception performance by combining the retransmitted data and previous data. The receiving terminal stores the received data in order to determine whether a burst (i.e. a set of data) received from the transmitting terminal corresponds to newly-transmitted data or retransmitted data, and to obtain a performance gain through combination of the data.
In order to normally perform the operation of the HARQ scheme, it is necessary to control the received HARQ burst, and to efficiently control and manage a memory which stores the received HARQ burst.
According to the Institute of Electrical and Electronic Engineers (IEEE) 802.16 standard and Mobile WiMAX standard, which are developing a wireless standard in relation to the HARQ scheme, the allocation of an HARQ burst by a transmitting terminal and the transmission of an ACK/NAK ReSPonse (ACK_RSP) to the HARQ burst by a receiving terminal are performed based on HARQ channels (i.e. HARQ Channel Identification (ACID), which is identification information for identifying HARQ channels). Also, whether a downlink HARQ burst corresponding to a newly-transmitted burst or a retransmitted burst is determined based on whether an HARQ Indicator Sequence Number (AI_SN) field information of an HARQ DownLink MAP (DL_MAP) message has been toggled to “0” or “1.”
That is, when the AI_SN field information upon allocating an HARQ burst to a corresponding ACID and AI_SN field information for current HARQ burst allocation are equal to each other, a retransmission has occurred, and when they are different, a new transmission has occurred.
Therefore, in order to combine allocated HARQ bursts according to ACIDs in the wireless communication system, a correct determination of new transmission and retransmission must first be performed. However, in the actual communication environments, there is a problem in that when retransmission is determined with only information about whether 1-bit AI_SN field information in the DL-MAP is toggled or not, the accuracy of the determination is low.                1) The following parameters, which are values used in the IEEE 802.16e standard, are items representing the performance of a terminal, and are values subjected to Subscriber station Basic Capacity (SBC) negotiation upon cooperation with a base station.        number of DL HARQ channels        DL HARQ buffer capability per channel        aggregation flag for DL        maximum number of DL HARQ bursts per frame        
In contrast, the following parameters are items defined by the base station and informed from the base station, and are values having no connection to the performance of the terminal.                ACK delay value for DL HARQ burst        maximum number of retransmission in DL HARQ        
In line with this, the Mobile WiMAX classifies and defines categories of parameters connected with the performance of a terminal. Generally, as a higher-level category is employed, a mean throughput supportable in a terminal increases, but a memory region required in the terminal also increases.
Therefore, when it is assumed that 4 bits are employed for a Log-Likelihood Ratio (LLR), the sizes of the buffers required for each category may be calculated as follows.
DL Category 1 (Aggregation ON/OFF)16,384 bits×4 (LLR bits)×4 channels=262,144 bits
DL Category 2 (Aggregation ON)8,192 bits×4 (LLR bits)×16 channels=524,288 bits
DL Category 3 (Aggregation ON)16,384 bits×4 (LLR bits)×16 channels=1,048,576 bits
DL Category 4 (Aggregation ON)23,170 bits×4 (LLR bits)×16 channels=1,482,880 bits
As described above, generally, as the level of a category increases, the size of a memory required for supporting the HARQ also increases. Especially, for categories 2 to 4, aggregation for an HARQ buffer is required as an essential item (Aggregation ON).
Particularly, when aggregation for an HARQ buffer is supported as described above, the conventional method allocates an allocatable maximum memory region according to each ACID in order to reduce the complexity caused by managing memory regions allocated according to ACIDs.
According to such a conventional method, since the management of only a start address and an end address according to each ACID is required, the complexity in implementation is reduced. However, according to such a memory management method, since a very large memory region is allocated to each ACID, the total of actually-required memory regions increases in geometric progression according to the number of used channels. That is, since an HARQ buffer is allocated using a start address and an end address according to each ACID, each ACID occupies a memory region of a predetermined size, so that it is possible to allocate and use each ACID within a corresponding memory region. However, an actual memory size calculated according to each category of the Mobile WiMAX requires a memory region twice as large as the number of HARQ channels. That is, a memory region of 1.0 Mbits is required for Category 1, 8.4 Mbits for Category 2, 16.8 Mbits for Category 3, and 23.7 Mbits for Category 4.
Therefore, it is necessary to develop a more efficient memory management method than the conventional method of allocating a fixed memory region according to each ACID. In addition, it is necessary to develop a new memory allocation method by taking into consideration the fact that a fixed memory region according to each used ACID increases in geometric progression when aggregation is supported.